1. Field of Invention
The present invention relates generally to a control system and method for a hybrid electric vehicle (HEV), and specifically to a strategy to control and transition between operating states in a parallel HEV.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and pollutants from automobiles and other vehicles powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors have attempted to address these needs. However, electric vehicles have limited range and limited power coupled with the substantial time needed to recharge their batteries. An alternative solution is to combine both an ICE and electric traction motor into one vehicle. Such vehicles are typically called hybrid electric vehicles (HEV""s). See generally, U.S. Pat. No. 5,343,970 to Severinsky.
The HEV has been described in a variety of configurations. Some HEV patents disclose systems where an operator is required to select between electric and internal combustion operation. In other configurations the electric motor drives one set of wheels and the ICE drives a different set.
Other, more useful, configurations have developed. A series hybrid electric vehicle (SHEV) is a vehicle with an engine (most typically an ICE), which powers a generator. The generator, in turn, provides electricity for a battery and motor coupled to the drive wheels of the vehicle. There is no mechanical connection between the engine and the drive wheels. A parallel hybrid electrical vehicle (PHEV) is a vehicle with an engine (most typically an ICE), battery, and electric motor combined to provide torque or power the wheels of the vehicle.
A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both the PHEV and the SHEV. The PSHEV is also known as a torque (or power) splitting powertrain configuration. Here, the torque output of the engine is given in part to the drive wheels and in part to an electrical generator. The generator powers a battery and motor that also provide torque output. In this configuration, torque output can come from either source or both simultaneously. The kinetic energy of the vehicle can be captured by the generator where it is converted to a charge that is sent to the battery (regenerative braking).
The desire of combining the ICE with an electric motor is clear. The ICE""s fuel consumption and pollutants are reduced with no appreciable loss of performance or vehicle range. A major benefit of parallel HEV configurations is that the engine can be turned off during periods of low or no power demand from the operator (e.g., waiting for a traffic light). This improves fuel economy by eliminating wasted fuel used during idle conditions. The motor can then propel the vehicle under conditions of low power demand. In most configurations, the engine can be disconnected from the motor and powertrain when it is not running by opening a disconnect clutch. As power demand increases, the engine can be restarted and reconnected to provide the requested torque.
Developing a strategy to control a parallel HEV is needed for successful implementation of a parallel HEV. HEV control strategies are known in the prior art. For example, U.S. Pat. No. 5,984,033 to Tamagawa et al. proposes a pre-transmission hybrid electric vehicle but there is no means to disconnect the internal combustion engine from the electric motor. This limits the vehicle operation and goals for a successful HEV strategy. Further, Tamagawa et al. does not provide a logical structure (such as a state machine) for determining different vehicle operating modes and only three modes of operation are allowed: assist, regenerative, and zero output. Other modes of operation, including engine stopping and starting, are not included. U.S. Pat. No. 5,898,282 to Drozdz et al. describes a series HEV control system. The control strategy only describes three main operating modes: high load driving condition (HLDC), low load driving condition (LLDC), and regenerative braking (RB).
Other U.S. patents also cover limited HEV controllers. See generally, U.S. Pat. No. 4,407,132 (but has no strategy to optimize fuel consumption), U.S. Pat. No. 5,856,709 to Ibaraki et al. (but only for a post-transmission HEV configuration, i.e., motor/generator after the transmission in the vehicle powertrain); U.S. Pat. No. 5,873,426 to Tabata et al. (also a pre-transmission HEV configuration controller). Finally, see U.S. Pat. No. 6,583,599 issued to Phillips et al. on Jun. 24, 2003. This patent describes a parallel HEV configuration control system, but only has six possible vehicle states since it is not designed to control the latest parallel HEV configurations where the electric motor/generator is rated to propel the vehicle independent of the ICE and the ICE can be disconnected from the powertrain.
In summary, there is a need for a comprehensive controller for a parallel HEV that can add drive states that do not include use of the ICE and maximize vehicle fuel economy.
Accordingly, the present invention provides a control strategy for a parallel hybrid electric vehicle (HEV) configuration that has a logical structure defining main system operating modes (states) as well as the transition between the different states. The HEV is configured so that power from the engine and the motor can (independently) provide torque to the vehicle powertrain.
The present invention describes a vehicle system controller for a parallel HEV that has a state machine having a plurality of predefined states representing the vehicle operating modes; a set of rules defining logical relationships between each of the plurality of predefined states; a set of commands unique to each state supplied to the subsystem controllers to achieve desired vehicle functionality within the states; and a set of transition conditions between the plurality of predefined states.
The predefined states can be prioritized according to operator demands, energy management requirements, and system fault occurrences. In general, the system defines system fault occurrences as a first priority, operator demands as a second priority, and energy demands as a third priority. In the event system performance is being compromised, the system can also be configured to have system fault occurrences as a first priority, energy demands as a second priority, and operator demands as a third priority.
The present invention can also be configured to have at least one of a plurality of transition flags. Each transition flag represents a logical relationship associated with sensed vehicle operating status, operator demand, or system faults.
Additionally, in the present invention the plurality of predefined states representing operating modes can be configured to include a BLEED state, BOOST state, CHARGE state, REGEN LOW VEL state, REGEN HIGH VEL state, ENGINE DRIVE state, ENGINE START state, ENGINE STOP state, MOTOR DRIVE state, and OFF state.
Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures.